Devices for calculating and displaying the load supported by cranes, derricks, and the like, have long been used as operator aids in preventing unstable conditions or the overstressing of structural elements in the crane boom. This capability is particularly important in mobile cranes of the type having telescopingly extendable booms which can be slewed through the whole or part of a circle during normal operation. By comparing the load indication of an operating aid with the load rating tables supplied by the crane manufacturer for a specific crane and operating configuration, an operator can determine the relative stability of the crane. Typically, two methods of determining the load supported by a crane have been employed.
The first method involves the direct measurement of the actual weight of the load by devices such as tensiometers, strain gauges, and the like.
The second method involves the calculation of the total effective hook load, which is determined by first calculating the total turning moment of the boom and load about the boom pivot pin. By dividing the total turning moment by the horizontal radius of the load from the pivot pin, the total effective load can be calculated.
With both methods the actual load or total effective load can thus be determined and displayed to the operator who, upon referral to the load rating tables, can determine the amount of crane lifting capacity remaining at any given time.
A problem with prior schemes is that the operator, in order to take advantage of a load indicating operating aid, must continuously watch the load indication and refer to the rating tables supplied by the manufacturer. A disadvantage of this type of arrangement is that it distracts the operator's attention, which should be primarily focused upon the load and boom. Additionally, as the boom is extended and/or luffed, the geometrical configuration of the crane is changed, requiring the operator to refer to a different chart appropriate for that particular configuration. The probability of operator error is compounded by the fact that the published rating tables can be (and are often) confusing and subject to misinterpretation. If a fly or jib is in use, additional calculations must be made by the operator to determine his loading capacity.
Prior art devices such as those described in U.S. Pat. Nos. 3,913,690 and 4,063,649 to Hubbard et al., have eased some of the above described problems somewhat by providing "law generators" or hard wired electrical circuit equivalents to the load tables which must be plugged, one at a time, into the load moment computer for any given operating configuration. These circuits however, have the disadvantage of requiring removal and replacement each time the operator switches operating mode of the crane and/or adds a fly or jib.
Prior art operating aids as a whole, tend to detract from the operator's attention and introduce an inconvenience, which many operators overcome by simply ignoring or defeating the aid in deference to their own experience and judgment. As cranes become larger and more complex, however, this reliance upon operator judgment is becoming less and less desirable. Additionally, operating aids are called upon to supply more diverse types of information to the operator while permitting him to maintain his concentration on the load and boom. Another problem with the prior art devices arising from this increasing complexity is the prolonged set up and implementation time required. Existing units are typically extremely complex and may require several days to incorporate within a crane and fully calibrate. This complexity carries over into the operating arena wherein the operator must have a relatively high level of technical competence to fully utilize the aid provided him. Additionally, the commercially available prior art devices typically are extremely expensive.